The invention is a gelled aqueous composition which is capable of serving as the fluid medium utilized to prepare a gravel slurry to be injected down a wellbore against the face of an incompetent formation in an oil, gas, water or geothermal well to reduce the migration of sand into the wellbore from the formation when the well is put on production. The invention also relates to a method for treating such a wellbore in an incompetent formation by placing a gravel pack in such a wellbore utilizing the gelled composition as a carrying fluid which subsequently is reduced in viscosity, positioning the gravel in a desired location without causing a reduction in permeability of the formation.
"Gravel packing" is well known in the art of gas and oil well treatment. A number of basic techniques are known for placing such "gravel packs" in a wellbore. U.S. Pat. No. 3,498,380 (1970) describes a method for placing high density gravel packs carrying from 10 to 25 lbs. of particulate solids per gallon by adding thickening or gelling agents to the carrier fluid. Cellulosic materials have been utilized to thicken water based carrying fluids of this nature and to such fluids have been added acidic materials to "break", i.e. reduce the viscosity of, such thickened aqueous fluids; U.S. Pat. No. 3,892,275 (1975). Others have employed aqueous based gravel pack slurries which are thickened with various natural gums and cellulosic, which aqueous fluids contain at least one water-soluble fluoride salt which is converted to hydrofluoric acid in solution by an acid-generating material which is also present. In such systems, a weak acid/weak acid salt buffer system is employed; U.S. Pat. Nos. 3,828,854 (1974); 3,889,753 (1975); 3,868,996 (1975). Thickened fluids with pHs of less than about 1.5 are not generally described in these patents, however, except that a combination of ammonium fluoride and HCl is described in the '753 patent.
The utilization of aqueous fluoboric acid solutions for acidizing siliceous clay containing formations to stablize clays in such formations and also in conjunction with mud acid (HCl/HF) solutions is known: U.S. Pat. Nos. 4,151,878 and 4,151,879 (1979). Neither of these patents suggest that a gelled composition comprised of water, fluoboric acid, and a hydroxyethylcellulose gelling agent may be advantageously employed for gravel packing of wellbores.